1. Field of the Invention (Technical Field)
The present invention relates to an apparatus and method for laser velocity interferometry, which apparatus and method employ a fixed interferometer cavity and delay element.
2. Background Art
Laser velocity interferometry is a widely used method for measurement of shock wave phenomena. The method was first introduced by L. M. Barker & R. E. Hollenbach, "Laser Interferometry for Measuring High Velocities of Any Reflecting Surface." J. Appl. Phys. vol. 43, no. 11, (November 1972). Barker and Hollenbach employed an unequal leg Michelson interferometer in which monochromatic light reflected from a moving target is split by a beamsplitter mirror and traverses two unequal paths. Interference is produced by which Doppler-induced changes in the frequency of the light beam reflected off the moving target may be measured.
A major modification to the basic method was set forth in W. F. Hemsing, "Velocity Sensing Interferometer (VISAR) Modification," Rev. Sci. Instrum. vol. 50, no. 1, (January 1979). Hemsing employs a differential amplifier to sum both recombined beams from the beamsplitter. The advantages of this modification are a stronger signal, rejection of unwanted self-light from the target, and simplified data reduction.
U.S. Pat. No. 4,334,779, to Domey, et al., entitled Non-Contact Optical Apparatus for Measuring the Length or Speed of a Relatively Moving Surface, discloses laser velocimeter apparatus which measures length or relative speed of a moving object.
U.S. Pat. No. 4,101,199, to Christensen, entitled Radiation Beam Splitter, also discloses laser velocimeter structure comprising multiple lasers and prisms, but no interferometer.
U.S. Pat. No. 4,632,548, to Gunter, Jr., et al., entitled Protection Lens Scanning Laser Velocimeter System, likewise fails to disclose interferometer structure for velocity measurement.
U.S. Pat. No. 4,717,240, to Gilby, entitled Interferometeric Beamsplitter, discloses a mechanical resonator driven by optical energy.
U.S. Pat. No. 4,666,296, to Bailly-Salins, entitled Velocity Interferometer with Continuously Variable Sensitivity, while a true velocity interferometer, employs a water column delay element that is not of unitary assembly.
Laser velocity interferometry requires very precise alignment and adjustment of optical elements. In the prior art, interferometers are built on optical "bread-board" tables, using commercial mounts for the optical elements. Frequent adjustment by expert personnel is required to maintain high contrast of the interference signals.
The present invention solves this adjustment problem by use of a "fixed-cavity" design in which the critical interferometer cavity elements are aligned and then permanently bonded together during assembly, thus providing a high quality interferometer cavity with a fixed velocity sensitivity which requires no further adjustment. This enables operation by relatively unskilled personnel having only minimal training.